Add protected branch whitelists for merging

[Chri-s]

Fixes #3668. This pull request adds a second whitelist to protected branches. When the second whitelist is not used (not checked in the settings for a protected branch), everyone with write access to a repository can merge a pull request, just like before #3544. This is the default.

If the second whitelist is used, only users (or team members) in the second white list are allowed to merge pull requests to a protected branch.

I am not sure if I described this clearly on the settings page for protected branches. Here is a screenshot:

[codecov-io]

Codecov Report

Merging #3689 into master will decrease coverage by 0.01%.
The diff coverage is 35%.

@@            Coverage Diff             @@
##           master    #3689      +/-   ##
==========================================
- Coverage    35.8%   35.79%   -0.02%     
==========================================
  Files         287      288       +1     
  Lines       41496    41578      +82     
==========================================
+ Hits        14857    14881      +24     
- Misses      24453    24501      +48     
- Partials     2186     2196      +10

Impacted Files	Coverage Δ
models/migrations/migrations.go	2.58% <ø> (ø)	⬆️
modules/auth/repo_form.go	33.76% <ø> (ø)	⬆️
models/migrations/v59.go	0% <0%> (ø)
models/pull.go	51.2% <0%> (ø)	⬆️
models/branches.go	48.55% <37.8%> (-4.55%)	⬇️
routers/repo/setting_protected_branch.go	45% <66.66%> (+0.55%)	⬆️
modules/process/manager.go	76.81% <0%> (-4.35%)	⬇️

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[lunny]

But I think maybe pull request whitelist the same as push whitelist when push whitelist is checked but pull request whitelist not checked.

[lafriks]

@lunny but that would limit configuration possibilities. This way it is more configurable

[thehowl]

just to be clear; @thehowI is a copycat, not actually me